"The girl is clever."
One difference is that technically, 소년 isn't about a boy at all, even though it's used that way.
In this context, 소 means young/juvenile. And 년 means "year".
Technically speaking, therefore 소년 can and does mean "juvenile/young years" or any youth/juvenile. For example, a juvenile detention center is a "소년원". But on its own, 소년 is often used in reference to a boy.